The present invention relates to the downhole evaluation of formation fluids produced into a wellbore. More particularly, the invention relates to a real-time downhole multiphase flow evaluation apparatus and method which distinguishes capacitive and conductive fluids by exciting the fluids with distinct transmitters.
Wellbores are drilled into earth formations to produce hydrocarbon fluids from subsurface reservoirs. The formation fluids are produced at changing flowrates and comprise varying mixtures of hydrocarbons and water. Efficient production of hydrocarbons requires information regarding the production flowrates from specific locations within the wellbore, and the quantity of water produced with the hydrocarbon fluids. Production logging tools measure the instantaneous relative quantity ("holdup") and instantaneous velocity of the formation fluids in the wellbore as a function of time and location in the wellbore.
Logging tools have used capacitance sensors to evaluate formation fluids downhole in wellbores. A capacitance sensor comprises a transmitter electrode and a detector electrode oriented in a selected geometry. Capacitance sensors measure small changes in materials, typically resulting from impurities in the materials. If two capacitance sensors are separated by a known distance and the responses of the sensors to perturbations in a flowing medium can be correlated, the velocity of the medium can be calculated. Additionally, the magnitude of the electrical current traveling from the transmitter to the detector is proportional to the electrical admittance of the medium occupying the volume between these electrodes. Because hydrocarbons are essentially nonconductive and have an electrical permittivity twice that of air, hydrocarbons have a very low admittance particularly exacerbated by low measurement frequencies. Conversely, reservoir produced water is at least moderately saline and has a correspondingly high admittance.
In production logging of formation fluids, the electrical admittance of the formation fluids can vary more than six orders of magnitude. Although tools have been proposed to measure fluid velocities and holdups with capacitance sensor arrays, such systems do not provide satisfactory performance as a production logging tool.
One method and apparatus for measuring multiphase properties with a capacitance sensor array was described in European Patent Application No. 0510774A2 to DenBoer, wherein a plurality of capacitors were positioned vertically in a pipeline by placing a single electrode on one side of the fluid sample, and a segmented electrode on the other side of the fluid. Higher placed electrode segments identified the fluid level in the pipeline, and lower placed electrode segments measured the impedance of the liquid. The fraction of water in the liquid-filled part of the pipeline was determined by calculating the effective dielectric constant of the fluid from the capacitor impedance measurement, and was based on the theoretical relationship between the effective dielectric constant of an oil/water mixture and the ratio of oil in the water.
This system is not effective as a production logging tool because capacitance sensors have a very high input impedance and are susceptible to stray capacitance. When a metallic tool body and a capacitance sensor are immersed in an electrically conductive fluid there will be stray capacitance between the sensor and the tool electronics. For a production logging tool which must operate downhole in a wellbore, elimination of stray capacitance between tool electronics and metallic tool housings is difficult to accomplish.
Downhole sensor arrays have been constructed to measure the velocity and holdup of constituent fluids in a flowing multiphase fluid, however a single sensor does not provide stable and accurate results in both electrically conductive and nonconductive fluids. For an alternating current at a selected frequency, the electrical admittance of a fluid is a function of the conductivity, permittivity, and geometry of the fluid. Admittance comprises the reciprocal of impedance, and is measured as a ratio of current to voltage. At frequencies less than 10 megahertz, the admittance of moderately saline Water essentially comprises a conductance and the admittance of oil and gas essentially comprises a capacitance. At these lower frequencies, the conductive admittance of waters found in a wellbore is typically many orders of magnitude greater than the capacitive admittance of oil and gas, and the capacitance of oil is only twice that of gas. The magnitude of the current detected by one sensor will be proportional to the magnitude of the fluid electrical admittance between a transmitter electrode and the sensor. Accordingly, a single sensor that spans the range of admittance in the fluids of interest will have a limited resolution.
U.S. Pat. No. 5,736,637 to Evans et al. (1998) disclosed a system for evaluating multiphase flow downhole with a production logging tool. An array of capacitance sensors was combined with an array of conductivity sensors, and the mutually exclusive-outputs were multiplexed. This concept doubles the number of sensors and space required for containing measurement electronics. Construction of such a system is difficult to accomplish in logging tools small enough to traverse a wellbore. Additionally, the construction of conductance sensors in proximity to capacitance sensors inevitably creates stray capacitances which will degrade the capacitance measurement.
A need exists for an improved downhole sensor which can efficiently provide stable and accurate multiphase fluid evaluation in a wellbore as the fluid electrical conductivity and flow rate changes. The system should provide high measurement resolution while traversing the narrow confines of a wellbore and should withstand elevated wellbore temperatures and pressures.